Inability to produce aqueous polyurethane dispersions with ultra-high solid contents prevents their performance in many different applications. Aqueous polyurethane dispersions with low solid contents result in unacceptable levels of shrinkage upon drying, inability to incorporate higher levels of fillers into final products, and requiring longer times to dry. In addition, ultra-high solid content polyurethane dispersions facilitate lower shipping and storage costs and production reduction time per unit volume of materials.
U.S. Pat. No. 4,130,523 discloses aqueous polymer latexes produced by a process in which a portion of a forming latex in a reaction zone is continuously withdrawn from the reaction zone during formation of a stable seed latex and an intermediate latex, and the withdrawn latex is continuously fed back to the reaction zone during the formation of the final latex.
U.S. Pat. No. 4,456,726 discloses the method of making highly concentrated, bimodal, aqueous synthetic resin dispersions by the emulsion polymerization of ethylenically unsaturated monomers, in the presence of emulsifiers and free radical-forming initiators, by adding to a first latex containing a first dispersed synthetic resin and an aqueous phase, a second latex containing a second dispersed synthetic resin and an aqueous phase and a monomer phase containing a free-radically polymerizable monomer, and then polymerizing the monomer, the average size of the particles of said first resin differing by a factor between 2 and 15 from that of the particles of second resin, the total weight of the resins and monomers representing 100 parts by weight, and the total weight of the aqueous phases representing not more than 70 parts by weight.
U.S. Pat. No. 5,340,858 discloses final aqueous polymer dispersions that are obtainable by polymerizing radical polymerizable monomers with the addition of an aqueous dispersion of a starting polymer by the method of free radical aqueous emulsion polymerization.
U.S. Pat. No. 5,340,859 discloses an aqueous polymer dispersion which is obtainable by polymerizing monomers by the method of free radical aqueous emulsion polymerization with the addition of at least two starting polymer dispersions, of which one contains not only particularly finely divided but also coarsely divided polymer particles.
U.S. Pat. No. 5,350,787 discloses an aqueous polymer dispersion which is obtainable by polymerizing at least one radical polymerizable monomer by the method of free radical aqueous emulsion polymerization with the addition of an aqueous dispersion of a starting polymer.
U.S. Pat. No. 5,426,146 discloses an aqueous polymer dispersion which is obtainable by polymerizing radical polymerizable monomers other than vinyl or vinylidene halides by the method of free radical aqueous emulsion polymerization with the addition of an aqueous starting polymer dispersion having a certain diameter distribution of the starting polymer particles present therein by the stream addition process.
U.S. Pat. No. 5,496,882 discloses an aqueous polymer dispersion which is obtainable by polymerizing at least one radical polymerizable monomer by the method of free radical aqueous emulsion polymerization with the addition of an aqueous dispersion of a starting polymer.
U.S. Pat. No. 5,498,655 discloses an aqueous polymer dispersion which is obtainable by polymerizing radical polymerizable monomers other than vinyl or vinylidene halides by the method of free radical aqueous emulsion polymerization with the addition of an aqueous starting polymer dispersion having a certain diameter distribution of the starting polymer particles present therein by the stream addition process.
U.S. Pat. No. 5,624,992 discloses an aqueous polymer dispersion which is obtainable by polymerizing monomers by the method of free radical aqueous emulsion polymerization with the addition of at least one fine and at least one coarse aqueous starting polymer dispersion.
Despite the research efforts in developing ultra-high solid content polyurethane dispersions, there is still a need for ultra-high solid content polyurethane dispersions that provide reduced shrinkage upon drying, facilitate loading of additional fillers, and requiring relatively lesser amounts of time to dry. Furthermore, there is a need for a continuous process to make ultra-high solid content polyurethane dispersions that provide reduced shrinkage upon drying, facilitate loading of additional fillers, and requiring relatively lesser amounts of time to dry.